The proposed studies investigate the plasticity of the peripheral taste system. Specifically, the response of the system to cutting of the chorda tympani taste nerve under conditions of normal diet and under conditions of a NaCl-deficient diet will be evaluated in adult rats. Specific Aim 1 is to record neurophysiological responses from single geniculate ganglion neurons to evaluate responses to sodium salts on the regenerate side of the tongue after nerve cut and on the contralateral intact side. Changes in voltage sensitivity of taste cells and amitoride sensitivity will be assessed. Specific Aim 2 evaluates mechanisms responsible for enhanced sodium responsiveness in sodium-restricted rats on the side of the tongue contralateral to nerve section. The role of the immune system in this phenomenon will be studied by using immunocytochemistry to demonstrate the nature of macrophage activation in relation to the peripheral taste system contralateral to nerve cut. The role of neural activity in effects contralateral to nerve sections will be evaluated using the neural sodium channel blocker tetrodotoxin (TTX). Specific Aim 3 will assess the anatomical relationship between numbers of geniculate ganglion cells innervating single fungiform taste buds and sizes of taste buds. Fluorescent retrograde neuroanatomical tracers that label ganglion cells after taste bud application will be used. Specific Aim 4 studies the relationship between taste bud size, numbers of bud cells, and rates of proliferation of taste cells after nerve cut and regeneration, and under dietary sodium restriction. This will be accomplished by quantitative histological analysis and BrdU labeling of cells undergoing mitosis. Specific Aim 5 evaluates whether geniculate ganglion cell death occurs after chorda tympani section. Histological methods and multiple labeling, before and after nerve regeneration, will be the experimental approaches for this aim.